Fix QASM3 exporter unitary gate naming

[TSS99]

Summary

Rename exported OpenQASM 3 gate definitions that would otherwise use the bare symbol unitary.

Fixes #14310.

Root cause

The OpenQASM 3 exporter preserved the UnitaryGate name when emitting a gate definition, so it produced output such as:

 gate unitary _gate_q_0 {
   ...
 }
 unitary q[0];

Downstream consumers such as Qiskit Aer treat unitary as a special instruction name rather than a user-defined gate symbol, so the exported program could be misclassified and fail when reloaded.

Fix

• rename declared gate symbols derived from unitary to unitary_gate through the exporter symbol table
• keep the change scoped to exported gate declarations and call sites only
• add a regression test for nested synthesized unitary definitions
• add a release note for the user-visible QASM output change

Tests

• PYTHONPATH=$PWD python -m unittest -v test.python.qasm3.test_export
• PYTHONPATH=$PWD python -m black --check qiskit/qasm3/exporter.py test/python/qasm3/test_export.py
• PYTHONPATH=$PWD python -m ruff check qiskit/qasm3/exporter.py test/python/qasm3/test_export.py

Notes or limitations

• reno lint could not be completed in this checkout because the local repository hits a Dulwich missing-object error while scanning note history (KeyError: b'09bcf1597b54b73fef724dc3f02a6fae8edbdf1f').

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description: OpenAI Codex (GPT-5)
• I used the following tool to generate or modify code: OpenAI Codex (GPT-5)

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[jakelishman]

This is not a correct fix, because the root fault is not in the exporter but in the importer. The data model of Qiskit is not violated until an Instruction called unitary that is not a UnitaryGate is constructed, and Qiskit is not the only source of OpenQASM files, so the fix cannot be to write out an OpenQASM file differently.